The present invention relates broadly to frequency synthesizers, and in particular to a high speed phase locked loop microwave synthesizer.
In the present art, microwave synthesizers exist in two forms either direct or indirect. Direct synthesizers have the advantage of frequency acquisition in less than 10 microsecond tuning time but have the disadvantage of high spurious output levels. Indirect synthesizers produce no spurious signals in the output frequency, but are slow in acquisition time.
The previously utilized phase locked sources were tuned mechanically to some frequency and electronically sweep at a low rate over a very narrow band in order to lock the loop. Methods utilizing linearizers and preset voltages have been used to lock the loop. A disadvantage with this approach has been with the noise of the linearizer and the temperature dependence of the circuits. In addition, sweep circuits consisting of up/down counters have been employed with acquisition times ranging from milliseconds up to seconds. Consequently, present fast frequency hopping synthesizers employ direct synthesis techniques which comprise a combination of switching and arithmetic manipulations such as mixing, multiplication and division to provide the desired rapid frequency hopping. The resulting frequency signals generated by these latter named frequency hopping synthesizers have relatively poor spectral purity due to the difficulty of eliminating undesired spurious mixer products, and other problems which are associated with these synthesizers such as switch leakage. In addition, fast frequency hopping synthesizers using such a direct synthesis implementation tend to be large and expensive due to complex filtering, shielding and radio frequency interference elimination requirements. Fast frequency hopping synthesizers using phase locked loop techniques solve many of the aforementioned problems associated with synthesizers operating in accordance with direct synthesis techniques. However, standard phase locked loop synthesizers are generally handicapped in their use in fast frequency hopping synthesizers, as previously mentioned, because of the limited speed at which they permit transitions between frequencies to be made.